Alkanes + Haloalkanes

Question 1

What are alkanes?

Answer 1

Saturated hydrocarbons

Question 2

General formula alkanes

Answer 2

CnH2n+2

Question 3

General formula cycloalkanes

Answer 3

CnH2n

Question 4

What is petroleum/crude oil?

Answer 4

A mixture of hydrocarbons, mainly alkanes. Separated by fractional distillation

Question 5

How does fractional distillation work?

Answer 5

• crude oil vaporised at 350°C
• in fractionating column rises up trays
• largest don’t vaporise, run to bottom- residue
• negative temperature gradient
• alkanes have different chain lengths, different boiling points, condense at diff temperature and drawn off at diff levels

Question 6

Fractional distillation works because?

Answer 6

• more carbons in formula: greater intermolecular van der Waals’ forces
• greater if= more energy required to separate molecule= higher bp
• larger alkanes= higher bp

Question 7

What is cracking

Answer 7

Breaking long chain alkanes into smaller hydrocarbons. Involves breaking C-C bonds

Question 8

Why is cracking needed?

Answer 8

• Shorter chain (light) fractions e.g petrol, naphtha are in higher demand- more valuable
• excess larger hydrocarbons e.g bitumen cracked int9 more valuable shorter chain

Question 9

What is a fraction

Answer 9

A group of similar length hydrocarbon chain which have similar boiling points that are collected in fraction of same temperature

Question 10

Thermal cracking

Answer 10

• high temp: 450-900°C
• high pressure
• no catalyst
• produces lots of straight chain alkanes + alkenes (raw materials for chemical industry)

Question 11

Catalytic cracking

Answer 11

• high temp: 450°C
• moderate pressure
• zeolite catalyst
• high proportion branched alkanes +alkenes, cyclic + mainly aromatic hydrocarbons +motor fuels

Question 12

Why do alkanes make great fuels?

Answer 12

Burning a small amount releases huge amounts of energy. However, produces lots of pollutants

Question 13

Complete combustion

Answer 13

Oxidise alkanes + other hydrocarbons with plenty of oxygen- CO2 and H20
C3H8(g) + 5O2(g) —> 3CO2(g) + 4H2O(g)

Question 14

Incomplete combustion

Answer 14

• limited (not enough) oxygen
• produces carbon monoxide (CO) instead of or as well as CO2.
• soot (C) can be formed

Question 15

Catalytic converter

Answer 15

Fitted to exhaust system of car to remove pollutants (unburnt hydrocarbons and oxides of nitrogen)

Question 16

How does burning fossil fuels contribute global warming

Answer 16

• produces CO2: greenhouse gas
• absorb infrared, emit some of absorbed energy back to earth= greenhouse effect
• increase CO2 in atmosphere makes earth warmer

Question 17

Pollutants made by internal combustion engine

Answer 17

• unburnt hydrocarbons
• oxides of nitrogen (NOx)- due to high pressure + temp cause nitrogen + O2 to react
• these react in presence sunlight-form ground level ozone (O3), component smog
• catalytic converters remove unburnt hydrocarbons and oxides of nitrogen

Question 18

How is acid rain caused

Answer 18

• combustion of hydrocarbons containing sulfur leads to sulfur dioxide- air pollution
• in atmosphere dissolve in moisture, convert into sulfuric acid- acid rain

Question 19

How can sulfur dioxide be removed

Answer 19

Powdered calcium carbonate/CaO mix with H2O= alkaline slurry.
Flue gas mix with alkaline slurry- acidic sulfur dioxide react to form harmless salt

Question 20

What is a free radical

Answer 20

A particle with an unpaired electron

Question 21

How are free radicals formed

Answer 21

Covalent bond is split equally, giving one electron to each atom (homolytic fission)

Question 22

Substitution reaction

Answer 22

One atom in a molecule is replaced by another atom or group of atoms

Question 23

Free radical substitution

Answer 23

Involves breaking a carbon- hydrogen bond in alkanes

Question 24

Conditions needed for free radical substitution

Answer 24

-ultraviolet light: high energy to break strong covalent bonds (photodissociation)
-excess methane to reduce further substitution
-

Question 25

What are chlorofluorocarbons

Answer 25

Halogenoalkanes molecules which have all hydrogen molecules replaced by chlorine and fluorine atoms

Question 26

Electrophile

Answer 26

Electron pair acceptor

Question 27

Nucleophile

Answer 27

Chemical species that donates an electron pair to form a covalent bond

Question 28

Elimination

Answer 28

A reaction in which a molecule loses atoms or groups of atoms to form C=C

Question 29

Nucleophilic substitution

Answer 29

The reaction of an electron pair donor (the nucleophile) with an electron pair acceptor (the electrophile). Involves one species being replaced with another species.

Question 30

Free radical substitution

Answer 30

A photochemical reaction between halogens and alkanes to form halogenoalkanes.

Question 31

Ozone depletion

Answer 31

Chlorine atoms catalyse the decomposition of ozone and contribute to the hole in the ozone layer.

Question 32

How is ozone formed

Answer 32

Naturally when an oxygen molecule (O2) is broken down into two free radicals by uv radiation. Free radicals attack other O2, form ozone

Question 33

Why are CFCs banned?

Answer 33

• used as coolant gas in fridges, solvents, propellants in aerosols
• damage to ozone layer- prevent from absorb UV, lead to skin cancer
• alternatives= HFCs (hydrofluorocarbons) + hydrocarbons

Question 34

Polar bond

Answer 34

A covalent bond where the electrons are not distributed equally, cause molecule to have slight dipole- one end slightly + charged other slightly -